Results: Scientists as superheroes? Well, maybe, at least in their ability to "see" through subsurface soil and rock, by using a new technique for monitoring groundwater contamination that eliminates the need to drill wells. Scientists recently performed the first field demonstration of a minimally invasive monitoring approach for tracking subsurface biogeochemical changes accompanying the bioreduction of a uranium-contaminated aquifer. Their results showed that the approach, called surface spectral-induced polarization (SSIP), is both feasible and practical for remote monitoring of microbial activity stimulated during microbiological reduction.

The SSIP approach lets scientists track geochemical and mineralogical changes that occur when electron donors, such as acetate, are added to groundwater to stimulate subsurface microbial activity. SSIP was developed by scientists from Lawrence Berkeley National Laboratory, Pacific Northwest National Laboratory, the University of Bonn in Germany, and the University of California, Berkeley. The field demonstration took place at a former uranium mill tailings site near Rifle, Colorado.

Why it matters: Groundwater contamination by industry and nuclear weapons programs has spurred research into the use of microorganisms to facilitate remediation by isolating aqueous metals and radionuclides in forms in which they can't move. Much of the research has focused on microorganisms capable of immobilizing contaminants, such as uranium, after introducing organic carbon compounds, such as acetate, lactate and ethanol.

But understanding just how microorganisms alter their physical and chemical environment during bioremediation is hindered by the inability to adequately assess subsurface microbial activity over dimensions relevant to a field site, which can encompass areas and depths of tens to hundreds of meters. The SSIP field monitoring approach makes it possible to monitor the subsurface with very high spatial resolution-areas as small as 0.3 m-and without the need for groundwater wells.

"Similar to how non-invasive medical imaging has reduced the need for invasive, exploratory surgeries, geophysical monitoring techniques, such as SSIP, allow us to monitor large volumes of aquifer sediments without having to drill groundwater wells, which saves money and disturbs less land," said project lead Dr. Kenneth Williams, LBNL. "These methods are well suited for assessing the activity and end products of stimulated microbial activity and their relationship to contaminant remediation over long periods of time without relying on conventional and labor-intensive sampling approaches."

Methods: The research team used SSIP to monitor stimulated microbial activity in the Rifle aquifer while acetate is added. They injected variable frequency currents into the ground and measured resulting voltage potentials using electrodes embedded in the ground surface. The electrical response was dependent upon the predominant metabolic processes active in the subsurface at a given point in time.

The accumulation of mineral precipitates, such as iron sulfide, and electroactive ions, such as ferrous and hydrogen sulfide) altered the ability of fluids in the subsurface pore spaces to conduct electrical charge. This accounted for the anomalous electrical response and revealed the usefulness of such measurements for monitoring mineralogical and geochemical changes accompanying subsurface bioremediation.

What's next: SSIP may also be used to extend geochemical data from a few boreholes that provide valuable information about remediation effectiveness over large areas. This will thus require less monitoring while providing a high level of assurance that the remedial process is working as intended.

Future work will focus on collecting SSIP over a much wider range of frequencies (e.g. 0.05-500 Hz), testing the method for detecting naturally occurring zones of bioreduction, and specifying system requirements for widespread application.

Acknowledgments: Kenneth Williams, LBNL, leads geophysical monitoring research for the Rifle Integrated Field Research Challenge, which is part of the Environmental Remediation Science Program, Office of Biological and Environmental Research (BER), U.S. Department of Energy.

The research team includes Susan S. Hubbard, LBNL; Jennifer Druhan, and Jillian F. Banfield, UC-Berkeley; Evan Arntzen, Phil Long, Michael J. Wilkins and Lucie N'Guessan, PNNL; and Andreas Kemna, University of Bonn. Electron microscopy was carried out at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by DOE-BER and located at PNNL.